1. Field of the Invention
This invention relates to magnetostatic wave devices and particularly to mode filters for suppressing higher order magnetostatic wave propagation modes.
2. Description of the Prior Art
Magnetostatic waves, which propagate in a supporting medium such as yttrium iron garnet (YIG) films, have potential applications in microwave delay lines and filters for use in radar, electrical countermeasure equipment and communication systems. Constant delay lines with bandwidths of 400 megahertz and delays of about 100 nanoseconds and linear dispersive "chirp" delay lines with bandwidths of about 1 gigahertz and differential delays of 200 nanoseconds have been demonstrated at 9 gigahertz. For example, a magnetostatic wave delay line which includes an adjustable dispersion characteristic is described in U.S. Pat. No. 3,935,550 which issued on Jan. 27, 1976 to John D. Adam, the inventor herein and Jeffrey H. Collins entitled "Group Delay Equalizer". In U.S. Pat. No. 3,935,550 a thin ferrimagnetic film such as yttrium iron garnet (YIG) is shown deposited on a non-magnetic substrate with an input transducer or coupler positioned at one end of the film for generating magnetostatic waves in the film in response to electrical signals at frequencies as high as 3 gigahertz. The generated magnetostatic wave propagates along the film in a direction transverse to the input transducer and is sensed by an output transducer positioned some distance from the input transducer along the YIG film. Several magnetic biasing arrangements are shown. A forward surface wave is generated by the input transducer if the H field is applied in the plane of the film but perpendicular to the direction of propagation. A forward volume wave is generated by the input transducer if the H field is normal to the plane of the film. A backward volume wave is formed by an input transducer when the H field is in the plane of the film in the direction of propagation of the magnetostatic wave. Typically a magnetostatic wave is generated by passing current through a wire or conductor placed adjacent to the YIG film. The magnetic field surrounding the wire induces the magnetostatic wave in the YIG film which then propagates in the YIG film depending on the magnitude and orientation of an external magnetic field resulting in a biasing H field in the film.
A more complex input transducer is described in U.S. Pat. No. 4,199,737 which issued on Apr. 22, 1980 to Ralph W. Patterson, Terence W. O'Keefe and John D. Adam, the inventor herein, entitled "Magnetostatic Wave Device". In U.S. Pat. No. 4,199,737 two sets of interdigitated fingers are connected to receive the same microwave input signal which is divided between the two sets. For example, FIG. 8 shows that each set has 5 fingers which are interdigitated with a second set of 5 fingers. FIG. 20 shows a set of 11 fingers interdigitated with a second set of 11 fingers wherein the lengths of the fingers are varied so that only some of the fingers are in interdigital relationship. The more complex input and output transducers permitted transducers to be made having predetermined spectral responses such as a narrow band response with low sidelobe levels and a flat peak response.
One of the problems still present with magnetostatic devices arises from the possibility of multi-mode propagation of the magnetostatic wave leading to attenuation and phase ripple on delay lines and undesired responses in bandpass filters.
A YIG film of finite width is effectively a magnetostatic waveguide and, in common with electromagnetic and acoustic waveguides, can support the usual desired lowest order mode plus higher order modes. With magnetostatic surface waves, only higher order "width" modes can exist but with magnetostatic volume waves both higher order "thickness" and "width" modes can exist. Higher order modes are launched, along with the lowest order mode, by the input transducer with the launching efficiency dependent upon the transducer geometry. Higher order modes of magnetostatic waves are also produced by scattering of the lowest order mode by defects in the YIG film. At the receiving transducer the highest order modes interfere with the lowest order mode, producing amplitude and phase ripple as well as increasing the overall insertion loss of the delay lines.
Higher order volume wave "thickness" modes generally have a delay which is more than twice as long as the lowest order mode and thus have higher insertion losses and consequently are less troublesome than the "width" modes. "Width" modes show longer delays than the lowest order mode at low frequencies, i.e., low wave numbers, but become degenerate with the lowest order mode at high frequencies. The width modes do not dissipate at the edges of the YIG film since no absorbing material is located there. It is understood that the width modes are traveling in the desired propagation direction with a variation in amplitude arising from interference between two waves propagating transverse to the desired propagation direction.
Electromagnetic waves, as compared to magnetostatic waves, have been known to have higher order modes of propagation which are propagating simultaneously in a waveguide. Higher order modes of electromagnetic waves have been attenuated by using mode filters which may for example consist of sheets of resistive material positioned in the waveguide. The use of mode filters in rectangular waveguides is described in a publication entitled "Mode Filters for Oversized Rectangular Waveguides" by Hans-Jurgen Butterweck appearing in IEEE Transactions on Microwave Theory and Techniques, Volume MTT-16, No. 5, May 1968, pp. 274-281. In FIG. 5 at page 278 a slotted waveguide is shown having slots in the upper and lower surface of the waveguide to interrupt currents traveling transverse on the upper and lower surface with resistive sidewalls for higher order mode attenuation. A second publication discussing mode filters is entitled "A Method of Calculating the Attenuation Constants of the Unwanted Modes in Mode Filters Using Resistive Sheets" by Sadakuni Shimada which appeared in the IEEE Transactions on Microwave Theory and Techniques, Volume MTT-14, pp. 159-161, 1966. In the publication, mode filters are provided by mounting resistive sheets in hollow waveguides so that they are always perpendicular to the electric field of the main mode. While these two publications are directed to attenuating selected modes of electromagnetic wave energy propagating in waveguides, it is the closest known art for providing mode filters and suppressing higher order modes of electromagnetic wave energy.
It is therefore desirable to provide a mode filter for suppressing higher order modes of magnetostatic waves in magnetostatic wave devices.
It is further desirable to provide a mode filter for suppressing of higher order width modes in magnetostatic waves traveling in a supporting medium by depositing conductive segments transverse to the direction of desired propagation.
It is further desirable to provide attenuation of higher order width modes of magnetostatic waves in a YIG film by depositing thin aluminum strips adjacent one another extending from edge to edge across the YIG film transverse to the direction of desired magnetostatic wave propagation.